A majority of laser diode driver integrated circuits (ICs) for optical transmission can be broadly divided into two categories. The first category includes devices using direct modulation. This is generally used for short distance transmission in which a laser diode driver IC is used to directly drive a laser diode module with a drive current supplied by the laser diode driver IC. The second category includes devices which generally use external modulation. These are generally used for long-distance transmission in which, for example, an electroabsorption (EA) modulator driver IC is used to drive an electroabsorption (EA) modulator module. In both of these categories laser diode driver ICs can be found in both die forms in which the IC is assembled inside the laser module, and packaged forms in which the IC is assembled outside of the laser module and connected by a transmission line.
As a result of the operating frequencies of digital communication circuits, such as laser diode driver ICs, increasing beyond one gigahertz (GHz), lumped-element techniques for analyzing circuit behavior are no longer valid. As a result, scattering parameters, or S-parameters, have been developed for this purpose. S-parameters measurements become particularly important when the operating frequencies of laser diode driver integrated circuits are high enough that the size of circuit elements becomes a significant fraction, approximately one-tenth, of a wavelength of the transmitted signal. In addition, it is difficult to measure voltages and currents in order to obtain impedance measurements at frequencies in the microwave range.
In general, S-parameters are a measure of the ratio of reflected waves to incident waves delivered to a device to be measured. In particular, the S11 parameter, also referred to as the input return loss, is the measured ratio of the reflected wave from the device input to the incident wave on the device input. The S22 parameter, also referred to as the output return loss, is the measured ratio of the reflected wave from the device output to the incident wave on the device output. The S11 parameter can be related to the input impedance, and the S22 parameter can be related to the output impedance. Accordingly, S-parameters may be used to characterize the impedance properties of a device or transmission line.
Due to the need for ever increasing data transmission rates, the use of conventional laser diode driver integrated circuits results in numerous signal quality problems due to high frequency impedance effects. Thus there is a need for laser diode driver integrated circuits that provide improved output signal quality and reduced impedance mismatch at high frequencies of operation.